We studied the effect of changes in photosynthesis (A_net) on respiration, soluble sugars and carbohydrates in Populus deltoides saplings under controlled environmental conditions. Daily measurements of leaf respiration, stem CO₂ efflux, and root CO₂ efflux were made under constant temperature conditions in growth chambers. After a pretreatment period one of three treatments was applied: 1) increased atmospheric CO₂ 2) decreased day length and light intensity, or 3) continuous darkness.

Results/Conclusions

Increased A_net due to elevated CO₂ resulted in a sustained increase in respiration throughout the plant. Leaf respiration increased 50%, stem CO₂ efflux increased 135%, and root CO₂ efflux increased 16%. A significant increase in leaf soluble sugars was observed. Decreasing light intensity and day length resulted in an expected decrease in A_net and caused a rapid decrease in respiration throughout the plants. Leaf respiration decreased 43%, stem efflux decreased 78%, and root efflux decreased 74%. A significant decrease in leaf and root soluble sugars and leaf non-structural carbohydrates was observed. Total elimination of light had a similar effect, resulting in a 68% decrease in leaf respiration, a 65% decrease in stem CO₂ efflux a 73% decrease in root CO₂ efflux and a significant decrease in leaf and root soluble sugars and leaf and root carbohydrates. In all treatments, changes in the rate of respiration in all tissues occurred within the first day of treatment application, suggesting that respiration throughout the plant was strongly dependent on newly acquired carbohydrates. In addition, a diurnal pattern in root CO₂ efflux was observed throughout the experiment suggesting fluctuations in the rate of root respiration were occurring on a daily basis due to carbohydrate status.